1580s, "censorious, inclined to find fault," from critic + -al (1). Sense of "important or essential for determining" is from c. 1600, originally in medicine. Meaning "of the nature of a crisis, in a condition of extreme doubt or danger" is from 1660s; that of "involving judgment as to the truth or merit of something" is from 1640s; that of "having the knowledge, ability, or discernment to pass judgment" is from 1640s. Meaning "pertaining to criticism" is from 1741.
Related: Criticality (1756; in the nuclear sense, 1950); critically (1650s, "accurately;" 1815, "in a critical situation"). In nuclear science, critical mass is attested from 1940.